Correctly handle Object * arguments that were encoded as nullptr

[dsnopek]

Fixes godotengine/godot#86478
Fixes #1056

This is an alternative to PR godotengine/godot#87613, fixing the issue in godot-cpp rather than in Godot itself. I explained my reasons for wanting to fix it in godot-cpp in this comment.

This uses a ternary expression to match the code in the encode() method just below the convert() method this PR changes:

	_FORCE_INLINE_ static void encode(T *p_var, void *p_ptr) {
		*reinterpret_cast<const void **>(p_ptr) = p_var ? p_var->_owner : nullptr;
	}

I think having the symmetry between the encode/decode methods is nice, even though it leads to a very long line of code. :-)

This PR also adds an automated test which will crash on master, but succeed with this PR.

[AThousandShips]

This makes sense and is elegant, matches below as stated

[dsnopek]

I've updated the PR per @reduz's requests, as well as made the encode() methods consist with that change.

And I just posted PR godotengine/godot#90394 to make the same change in the equivalent code in Godot.

[reduz]

Looks great!

[dsnopek]

Thanks for the review!

[dsnopek]

Cherry-picked for 4.2 in PR #1465

[dsnopek]

Cherry-picked for 4.1 in PR #1466